Directional drilling apparatus

ABSTRACT

Directional drilling apparatus having a non-rotating mass rotatably a centre-of-gravity spaced form the mandrel axis. An offsetting arrangement is coupled to the mass, and has an outer profile defining an offset relative to the mandrel axis, and a bearing portion rotatably mounted on the offsetting portion. The apparatus is run on a string, and the offsetting portion is oriented relative to the mass. When the towards an orientation with its centre-of-gravity positioned towards the low side of the bore and tends to maintain the relative orientation.

FIELD OF THE INVENTION

[0001] The present invention relates to drilling apparatus, and inparticular to directional drilling apparatus. Embodiments of theinvention relate to directional drilling apparatus utilising aneccentric mass to maintain an offset in a drill string element, causinga drill bit to deviate in a desired direction. Other embodiments of theinvention relate to directional drilling apparatus utilising the mass ofthe drill string, and items and tools mounted on the drill string, tomaintain an offset.

BACKGROUND OF THE INVENTION

[0002] In directional or controlled trajectory drilling, the verticalinclination and azimuth of a drilled bore may be controlled such thatthe bore may extend from the surface to a target area which is notvertically aligned with the point on the surface where drillingcommences. This permits a wide area to be accessed from a singledrilling location and is therefore particularly useful in offshoredrilling operations.

[0003] Conventionally, rotation of the drill bit mounted on the lowerend of the drill string is achieved by rotation of the entire drillstring, by a rotating turntable or “top drive” on the surface, and oftenalso by a downhole motor located on the drill string adjacent the bit.The downhole motor is usually driven by the drilling fluid which ispumped through the string. Steerable downhole motors include a “bent”housing or elbow which introduces a small deviation (around 1°) in theend portion of the drill string. When the entire string is rotating suchan elbow has little or no effect on the bore trajectory. However, if thestring is stopped and then adjusted such that the motor bend is in adesired direction, rotating the drill bit using only the downhole motorwill result in the trajectory of the well deviating. However, progresswhen drilling in this manner, without rotation of the drill string,tends to be relatively slow.

[0004] Various attempts have been made to provide drilling apparatuswhich will permit bore trajectory to be varied or controlled while stillrotating the drill string, in some instances by providing a non-rotatingeccentric mass on the drill string adjacent the drill bit. In someproposals, the mass engages the “low” portion of the bore wall andsupports the drill string. A radially extending blade is mounted on themass and engages the bore to produce a lateral force on the drill stringcausing the drill bit to deviate from its existing path, or at leastprevents further deviation in the direction of the blade. However, thesuccess of such apparatus has been limited as the mass provides anunstable support for the heavy drill string, such that the mass islikely to topple and be moved to one side by the string, which will tendto move downwards to occupy the lower part of the bore.. Examples ofsuch arrangements are illustrated in U.S. Pat. Nos 4,638,873 and4,220,213.

[0005] WO96\31679 describes a surface controlled well bore directionalsteering tool comprising a mandrel for forming part of the drill string,and two eccentric sleeves. The outer sleeve has an eccentric bore thatforms a pregnant or weighted side. Two stabiliser shoes are provided oneither side of the sleeve at 90° to the pregnant housing. The innersleeve has a further eccentric longitudinal bore that contains themandrel. The relative orientations of the sleeves may be controlled tomove the mandrel to one side of the pregnant housing, thus transmittinga fulcrum force to the bit. An electric motor in the housing may beactivated from surface to rotate the inner sleeve. As far as the presentapplicant is aware, this tool has not been commercialised, and it isbelieved that the tool would prove difficult to operate.

[0006] Applicant's GD 2,343,470 and U.S. patent application Ser. No.09/435,453, and also WO97\47848 and U.S. patent application Ser. No.09/202,342, the disclosures of which are incorporated herein byreference, describe arrangements including non-rotating offset massesprovided in combination with pairs of stabilisers at either end of themass, one of the stabilisers being adjustable relative to the mass toprovide a desired offset of the drill string in the bore.

[0007] Other forms of directional drilling apparatus for controllinghole direction or inclination by providing eccentric or offset blades ormembers are described in U.S. Pat. Nos. 3,062,303, 3,092,188, 3,650,338,3,825,081 and 4,305,474. U.S. Pat. No. 6,216,802 describes anarrangement for orienting a drilling assemble featuring two drive shaftscoupled by a universal joint, with an orientation collar operable tochange the orientation of one of the drive shafts.

[0008] It is among the objectives of the embodiments of the presentinvention to provide improved directional drilling apparatus utilisingan offset or eccentric mass, or by offsetting the drill string itself.

SUMMARY OF THE INVENTION

[0009] According to one aspect of the present invention there isprovided a directional drilling apparatus for use in drilling a deviatedbore, the apparatus comprising:

[0010] a mandrel for mounting to a drill string and having a main axis;

[0011] a non-rotating mass rotatably mounted on the mandrel and having acentre-of-gravity spaced from the mandrel axis; and

[0012] an offsetting arrangement including a non-rotating offsettingportion rotatably mounted on the mandrel and having an outer profiledefining an offset relative to the mandrel axis, and a bearing portionrotatably mounted on the offsetting portion, the offsetting portionbeing coupled to the mass.

[0013] In use, with the apparatus located in a bore and the mandrelrotating, by locating or orienting the offset of the offsetting portionat a selected position, which location or orientation is maintained bythe mass, the mandrel is offset in the bore, which offset may beutilised to urge a drill bit coupled to the mandrel in a desireddirection.

[0014] The invention also relates to a directional drilling methodutilising such an apparatus.

[0015] The provision of a bearing portion which is rotatably mounted onthe offsetting portion serves to isolate the mandrel and offsettingportion from the bore wall, and thus facilitate operation of suchdirectional drilling apparatus. In testing, apparatus in accordance withan embodiment of the invention has avoided many of the difficultiesassociated with prior proposals, and in particular the tendency of theoffsetting portion and mass to rotate in the bore. In one test, anarrangement which had previously displayed a tendency to rotate the massevery 20 to 30 minutes, and thus disrupt the desired orientation,operated without difficulty when an offset stabiliser was provided withan appropriate bearing portion.

[0016] Although the particular mechanism which causes the rotation ofthe mass and offset stabilisers in prior proposals has not beenunambiguously identified, it is believed that the presence of thebearing portion substantially prevents the transfer of torque from therotating mandrel to an outer bearing surface of the bearing portionwhich may come into contact with the bore wall. In particular, althoughthe mass and offsetting portion are rotatably mounted on the mandrel,typically through appropriate bearing arrangements, there will still besome transfer of torque to the mass and the offsetting portion. Thus, inprior arrangements absent a bearing portion, it is believed there is atendency for the offsetting portion to “climb” around the bore wall.Thus, in certain situations this tendency, in combination with themoment arm created by offsetting the mandrel and attached drill stringto one side of the bore, may result in rotation of the offsettingportion and mass in the bore, and loss of the desired effect. Inpractice, this may result in an apparatus featuring an offset massoperating effectively when it is desired to deviate in one direction,typically to the left, but not operating as effectively when theapparatus is utilised to turn a bore to the right, when friction and theabove noted moment arm combine. Furthermore, with the present invention,any rotation of the bearing portion induced by other, externalinfluences, such as contact with the bore wall, is isolated from theoffset portion and mass.

[0017] Preferably, the apparatus is provided in combination with a drillbit.

[0018] Preferably, the non-rotating offsetting portion is coupled to themass to permit variation of the relative angular orientation of the massand offsetting portion, to achieve a desired deviation of the bit. Thisvariation may be achieved by any appropriate means, examples of whichare described in our GB 2,343,470 and WO97\47848. Most preferably, therelative angular orientation of the mass and offsetting portion iseffected by rotation of one of the mass and offsetting portion relativeto the other on the mandrel, and this relative rotation may be achievedby rotation of the mandrel. Preferably a selectively engageable geararrangement is provided between the mandrel and the mass or offsettingportion. Most preferably the gear arrangement comprises a harmonicdrive- This form of drive is very compact and allows for a relativelyhigh reduction ratio: in one embodiment, the reduction ratio is 160:1,that is one complete relative rotation of the offsetting portion or themass is achieved by rotating the mandrel, and the drill string to whichthe mandrel is coupled, 160 times. This facilitates accurate location ofthe offsetting portion without the provision of complex equipment, as anoperator simply has to count the number of rotations of the drill stringat surface to achieve a desired drilling direction. For example, in thisembodiment 40 rotations of the drill string will move the offsettingportion and mass 90° relative to one another, and any minor errors incounting the number of rotations will have no significant effect on theresulting drilling direction. This gearing also allows transfer ofsignificant torque to the mass or offsetting portion. Where the mass iscapable of rotation relative to the offsetting portion, the maximumtorque necessary to rotate the mass in the bore is known, such that thegear arrangement may be designed and built to deal with this torque.

[0019] The gear arrangement may be selectively engaged or disengaged inresponse to any appropriate condition or signal, including appliedweight or tension, electrical or radio signals, and is most preferablyresponsive to fluid pressure. In a preferred embodiment, the mandrel ishollow to permit passage of drilling fluid, and the gear arrangement maybe responsive to drilling fluid pressure. Most conveniently, relativelyhigh fluid pressure, as experienced during the course of a drillingoperation, serves to disengage the gear arrangement. Alternatively, orin addition, the gear arrangement may be arranged to be locked out whiledisengaged, to prevent inadvertent relative rotation of the mass oroffsetting portion.

[0020] Preferably, the coupling between the offsetting portion and themass provides for a datum set position, in which the relativepositioning of the offsetting portion and the mass is known. Thispermits accurate relative location of the portion and mass withoutrequiring complex sensors and transmitters, as the portion and the massmay be located in the datum set position and then moved relative to oneanother to a desired angular orientation. When provided in combinationwith a gear arrangement between the mandrel and one or both of theoffsetting portion and the mass, it is preferred that the geararrangement is adapted to disengage on the portion and mass reaching thedatum set position. Furthermore, it is preferred that the geararrangement may then be engaged to permit the angular orientation of themass and offsetting portion to be set. In a preferred arrangement thisis achieved by a pressure pulse achieved by, for example, turningdrilling mud pumps on and then off.

[0021] Alternatively, or in addition, an orientation sensor may beprovided for the offsetting portion, which sensor transmits signals tosurface indicative of offsetting portion position. The signals may betransmitted directly to surface, or via another tool, such as ameasurement while drilling (MWD) tool or a logging while drilling (LWD)tool; the MWD tool typically converts inputs to drilling fluid pulses,which may detected and interpreted at surface.

[0022] The non-rotating portion of the stabiliser may comprise aplurality of parts, and in one embodiment comprises inner and outerparts, each part defining an offset bore: by varying the relativeorientation of the offset bores it is possible vary the offset of themandrel relative to the outer circumference of the stabiliser. With suchan arrangement it is possible to configure the stabiliser such that itdoes not create an offset, allowing a bore to be drilled straight ahead.

[0023] A relatively flexible string portion may be located adjacent saidoffsetting means. In certain circumstances, as described in GB 2,343,470, the provision of a relatively flexible string portion, such asa flex joint, in the string adjacent the offsetting means, facilitatesaccommodation of the deviation that may be introduced in the bore byoperation of the offsetting means.

[0024] Preferably, the mass is selected to describe a smaller diameterthan the bore. Thus, the mass is normally maintained clear of the borewall, obviating any tendency for the heavy drill string to rest on themass and topple to one side of the mass. Conveniently, the mass may bemounted between two larger diameter string elements, such asstabilisers, one of which comprises the offsetting arrangement, and bothof which are preferably non-rotating. Most preferably, the offsettingarrangement is located between the mass and the drill bit, although inother embodiments the offsetting arrangement may alternatively be spacedfrom the bit by the mass, or may be spaced from the bit by a stabiliserwhich acts as a fulcrum. In the preferred arrangement, with the primaryoffsetting arrangement located between the mass and the drill bit, theother larger diameter string element at the opposite end of the mass mayalso provide an offset. The location or orientation of the offset may bevariable or adjustable, however in the preferred arrangement the offsetis arranged to locate the mandrel or drill string towards the low sideof the bore, while still maintaining the mass clear of the bore wall.Such an offset makes little difference to the drilling direction butassists in maintaining the desired orientation of the primary offsettingarrangement by locating the mandrel and drill string to the low side ofthe bore. Indeed, in some instances such offsetting of the drill stringmay be sufficient to maintain a desired offset without requiringprovision of an eccentric mass, which invention is the subject ofanother aspect of this invention.

[0025] Preferably, a rotating cutting arrangement, such as a rotatingstabiliser provided with suitable cutting faces, or some other cuttingarrangement, is provided adjacent the drill bit, and preferably betweenthe first non-rotating element and the drill bit. When the drill bit isurged in a different direction by a change in relative orientation ofthe mass and offsetting portion, the bit may initially movepredominately laterally, creating a ledge. If a non-rotating element,such as a non-rotating stabiliser, then encounters this ledge, theoffsetting effect produced by the apparatus may be exaggerated, suchthat the drill bit creates a further ledge. Ultimately, this ledgeformation process may result in the drilling apparatus being unable toproceed further. The provision of a rotating cutter adjacent the bit,which is slightly undergauge, for example by ⅛″, and preferably ofsimilar dimensions to the following non-rotating stabiliser, reamsthrough the bore after the drill bit and removes any ledges formed bythe bit, creating an opening through which the following non-rotatingstabiliser may pass. This allows deviation of the bore at a controlledrate, typically 3°/100 feet.

[0026] According to another aspect of the present invention there isprovided a directional drilling apparatus for use in drilling a deviatedbore, the apparatus comprising:

[0027] a mandrel for mounting to a drill string and having a main axis;

[0028] a non-rotating mass rotatably mounted on the mandrel and having acentre-of-gravity spaced from the mandrel axis;

[0029] an offsetting arrangement including a non-rotating offsettingportion rotatably mounted on the mandrel and having an outer profiledefining an offset relative to the mandrel axis;

[0030] means for selectively coupling the offsetting portion to themass; and

[0031] a gear arrangement for selectively coupling at least one of themass or offsetting portion to the mandrel, whereby rotation of themandrel is translatable to a lesser degree of rotation of one of themass or offsetting portion.

[0032] According to a still further aspect of the present inventionthere is provided a directional drilling apparatus for use in drilling adeviated bore, the apparatus comprising:

[0033] a mandrel for mounting to a drill string and having a main axis;

[0034] a non-rotating mass rotatably mounted on the mandrel and having acentre-of-gravity spaced from the mandrel axis;

[0035] an offsetting arrangement including a non-rotating offsettingportion rotatably mounted on the mandrel and having an outer profiledefining an offset relative to the mandrel axis, the offsetting portionbeing coupled to the mass; and

[0036] a offsetting portion position sensor providing an offset positionoutput signal.

[0037] According to a yet further aspect of the present invention thereis provided a directional drilling apparatus for use in drilling adeviated bore, the apparatus comprising:

[0038] a mandrel for mounting to a drill string and having a main axis;

[0039] a drill bit coupled to a leading end of the mandrel;

[0040] a non-rotating mass rotatably mounted on the mandrel and having acentre-of-gravity spaced from the mandrel axis;

[0041] an offsetting arrangement including a non-rotating offsettingportion rotatably mounted on the mandrel and having an outer profiledefining an offset relative to the mandrel axis, the offsetting portionbeing coupled to the mass; and

[0042] a near-gauge rotating cutter located between the drill bit and aleading non-rotating element of the apparatus, whereby, in use, thecutter removes ledges created by changes in direction of the drill bit.

[0043] According to an alternative aspect of the present invention thereis provided a directional drilling apparatus for use in drilling adeviated bore having a low side, the apparatus comprising:

[0044] a mandrel for mounting to a drill string and having a main axis;

[0045] a primary offsetting arrangement including a non-rotatingoffsetting portion rotatably mounted on the mandrel and having an outerprofile defining an offset relative to the mandrel axis; and

[0046] a secondary offsetting arrangement including a non-rotatingportion coupled to the primary offsetting arrangement and arranged tooffset the mandrel towards the low side of an inclined bore.

[0047] According to another aspect of the present invention there isprovided a method of drilling a deviating bore, the method comprisingthe steps:

[0048] mounting a drill bit on a drill string and locating the string ina bore having a low side;

[0049] rotating the drill string such that the bit rotates;

[0050] providing a primary non-rotating offsetting arrangement towardsthe leading end of the string;

[0051] offsetting the leading end of the string in the bore such thatthe bit tends to advance in the direction of the offset;

[0052] providing a secondary non-rotating offsetting arrangement on aportion of the drill string following said leading end and coupled tosaid primary non-rotating offsetting arrangement; and

[0053] offsetting said portion of the drill string towards the low sideof the bore such that the mass of said portion of the string tends tomaintain the offset of said leading end.

[0054] These two aspects of the present invention operate in a similarfashion to the previous aspects, however rather than maintaining theprimary offsetting arrangement in a desired position by utilising anon-rotating mass, the mass of the drill string, and in particular thetendency for the drill string to want to lie towards the low side of thebore, is utilised to maintain the desired offset.

[0055] As used in relation to the latter aspect, the term drill stringis intended to include, where appropriate, not only drill pipe and thelike, but any mandrel forming part of the apparatus, and any items suchas drill collars, subs and MWD tools mounted on or to the drill pipe.

[0056] Those of skill in the art will realise that many of the variouspreferred and alternative features described above with reference to thefirst aspect of the invention may also be provided, singly and incombination, in combination with the other aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] These and other aspects of the present invention will now bedescribed, by way of example, with reference to the accompanyingdrawings, in which:

[0058]FIG. 1 is an elevation of directional drilling apparatus for usein drilling a deviated bore in accordance with an embodiment of thepresent invention:

[0059]FIG. 2 is an enlarged sectional view on line 2-2 of FIG. 1;

[0060]FIG. 3 is an enlarged sectional view on line 3-3 of FIG. 1;

[0061]FIG. 4 is an enlarged sectional view on line 4-4 of FIG. 1;

[0062]FIG. 5a is an enlarged sectional view of a gearing arrangement ofthe apparatus of FIG. 1, showing the gearing arrangement in a firstconfiguration;

[0063]FIG. 5b is a development of a part of the arrangement of FIG. 5a;

[0064]FIG. 6a is an enlarged sectional view of a gearing arrangement ofthe apparatus of FIG. 1, showing the gearing arrangement in a secondconfiguration;

[0065]FIG. 6b is a development of a part of the arrangement of FIG. 6a;

[0066]FIG. 7a is an enlarged sectional view of a gearing arrangement ofthe apparatus of FIG. 1, showing the gearing arrangement in a thirdconfiguration;

[0067]FIG. 7b is a development of a part of the arrangement of FIG. 7a;

[0068]FIG. 8a is an enlarged sectional view of a gearing arrangement ofthe apparatus of FIG. 1, showing the gearing arrangement in a fourthconfiguration;

[0069]FIG. 8b is a development of a part of the arrangement of FIG. 8a;and

[0070]FIG. 9 is an enlarged sectional view of an offsetting arrangementof the apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

[0071] Reference is first made to FIG. 1 of the drawings, whichillustrates a directional drilling apparatus for use in drilling adeviated bore, in accordance with a preferred embodiment of the presentinvention. The apparatus 10 is mounted to the lower end of a drillstring 12, formed of drill pipe sections, and includes a mandrel 14having a following end coupled to the drill string 12 and a leading endcoupled to a rotating stabiliser 16, with a drill bit 18 being mountedto the stabiliser 16. Rotatably mounted on the mandrel 14 are a primaryoffset stabiliser 20, an eccentric mass 22, and a secondary offsetstabiliser 24. Accordingly, in use, during a drilling operation, thedrill string 12 is rotated from surface, which in turn rotates themandrel 14, stabiliser 16 and drill bit 18. However, the offsetstabilisers 20, 24 and the mass 22 are intended to remain substantiallystationary in the bore, other than to advance axially with the rest ofthe apparatus, that is the stabilisers 20, 24 and the mass 22 do notrotate.

[0072] Before describing the apparatus 10 in detail, the operation ofthe apparatus 10 will be briefly described. The apparatus 10 is utilisedin directional drilling and permits the drill bit 18 to be directed todrill in a selected direction; to the side, upwards or downwards. Thisis achieved by arranging the primary offset stabiliser 20 to offset themandrel 14, and thus the drill bit 18, in the bore towards the desireddrilling direction. The desired offset or orientation of the stabiliser20 is maintained by coupling the stabiliser 20 to the mass 22, whichfeatures a centre of gravity spaced from the mandrel axis, such that themass 22 tends to lie towards the low side of the bore. This effect isenhanced by the provision of the secondary offset stabiliser 24, whichoffsets the mandrel 14 towards the low side of the bore, such that theweight of the mandrel 14, drill string 12, and any apparatus and toolsmounted on the drill string 12, similarly contribute to maintaining thedesired offset of the stabiliser 20.

[0073] The orientation of the offset provided by the stabiliser 20, andthus the drilling direction, may be varied by changing the relativeorientation of the stabiliser 20 and the mass 22. This variation inorientation of the offset stabiliser 20 is achieved by means of aharmonic drive gear assembly 26 which may be configured such thatrotation of the drill string 12 and mandrel 14 is translated to rotationof the stabiliser 20 relative to the mass 22.

[0074] Reference is now made to FIG. 2 of the drawings, which is anenlarged cross-sectional view of the primary offset stabiliser 20. Itwill be observed that the stabiliser 20 is mounted on the mandrel 14,and that the stabiliser 20 includes a non-rotating offsetting portion 28having an outer profile 30 defining an offset relative to the mandrelaxis, with the result that the stabiliser main axis or centre line 32 isspaced from the mandrel main access or centre line 34. A bearing portion36, defining helical blades 38, is rotatably mounted on the offsettingportion 28, via needle roller bearing 39, and serves to isolate anytorque transferred from the rotating mandrel 14 to the non-rotatingoffsetting portion 28 from the bore wall.

[0075] Reference is now made to FIG. 3 of the drawings, which is across-section of the mass 22 and illustrates the configuration of themass, which results in the centre of gravity of the mass being offsetfrom the mandrel centre line 34, with the result that the mass 22 tendsto lie in the configuration shown, that is with the “heavy” side of themass 22 lying to the low side of the bore.

[0076] Reference is now made to FIG. 4 of the drawings which is across-sectional view of the secondary offset stabiliser 24. It will benoted that the stabiliser 24 is of a substantially similar configurationof the stabiliser 20, having a non-rotating offsetting portion 40 and abearing portion 42 rotatably mounted on the offsetting portion 40.However, in this embodiment the stabiliser provides a fixed ({fraction(3/16)}″) offset to the low side of the bore.

[0077] Reference will now be made to FIGS. 5 through 8 of the drawings,which illustrate the harmonic drive gear assembly 26 in greater detail.Reference is first made to FIG. 5a of the drawings, which illustratesthe relative positioning of the elements of the gear assembly 26 duringdrilling, with the relative locations of the stabiliser 20 and mass 22fixed and the mandrel 14 rotating freely relative to the stabiliser 20and mass 22. The figure illustrates the mandrel 14 passing through theassembly 26, which includes sleeves 20 a, 20 b forming part of the.stabiliser 20, and a sleeve 22 a which is coupled to the mass 22.

[0078] The gear assembly 26 of course includes the elements of theharmonic drive, including an inner drive gear ring 44 and outer drivengear cups 46, 48 which are rotatably coupled to the mass sleeve 22 a andthe stabiliser sleeve 20 b, respectively. Located between the drive gearring 44 and the outer driven gear cups 46, 48 is a toothed belt 50. Thegear ring 44 includes a slight ovality and the outer driven gears cups46, 48 have a different number of teeth, such that rotation of the drivering 44, transferred via the belt 50, results in relative rotation ofthe outer driven cups 46, 48, and thus rotation of the mass 22 relativeto the stabiliser 20. The driven ring 46 is coupled to the mass sleeve22 a by dog gears 52, while the driven ring 48 is coupled to thestabiliser sleeve 20 b by a pin and slot arrangement 54.

[0079] In the configuration as illustrated in FIG. 5a, it is theintention that there should be no relative rotation between thestabiliser 20 and the mass 22, and the gear assembly therefore includesa gear locking mechanism comprising a locking sleeve 56 which isrotatably coupled to the stabiliser sleeve 20 b and which is urged by aspring 58 to engage the drive ring 44 via dog gears 60.

[0080] Rotation of the mandrel 14 is transferred to the drive ring 44via a pressure responsive sleeve 62 mounted on the mandrel 14. However,during a normal drilling operation, when the mandrel bore 64 is occupiedby pressurised drilling fluid, fluid ports 66 in the mandrel wallcommunicate drilling fluid pressure to a piston 68 defined by the sleeve62 and urges the sleeve 62 into a position in which circumferentiallyspaced teeth 70 provided on the sleeve 62 are spaced from radiallyextending sprung dogs 72 provided in the drive ring 44. The lower end ofthe sleeve 62 features axial slots 74 which co-operate with pins 76formed on the mandrel 14, and which therefore allow transfer of rotationfrom the mandrel 14 to the sleeve 62. The upper end of the sleeves 62abuts, via a bearing 78, a collar 80 on the mandrel which carries asprung pin 82. The collar 80 is urged downwardly relative to the masssleeve 22 a by a spring 84, but during a drilling operation, and in thepresence of pressurised drilling fluid in the mandrel bore 64, thesleeve 62 pushes the collar 80 upwardly against the spring 84. Thesprung pin 82 mounted on the collar 80 extends radially through achannel 86 in the mass sleeve 22 a and into a slot 88 in the stabilisersleeve 20 b.

[0081] Reference is now made to FIG. 6a of the drawings, whichillustrates the configuration of the gear assembly 26 when the drillingfluid pumps have been shut down. In the absence of elevated drillingfluid pressure, the sleeve 62 is urged downwards by the spring 84 tolocate the sleeve teeth 70 in engagement with the drive ring dogs 72.Further, a pin 90 extending radially from the sleeve 62 engages the gearlocking sleeve 56, to disengage the dog gears 60. Furthermore, thesprung pin 82 moves to the lower side of the slot 88.

[0082] If the mandrel 14 is now rotated, the corresponding rotation ofthe sleeve 62 is transferred to the drive ring 44 and thus producesrelative rotation of the mass sleeve 22 a and the stabiliser sleeve 20b, such that the mass 22 will rotate relative to the stabiliseroffsetting portion 28.

[0083] In the illustrated embodiment the number of teeth on the drivering 44 and driven cups 46, 48 are selected such that one hundred andsixty rotations of the mandrel 14 will produce one complete (360°)rotation of the mass 22 relative to the stabiliser 20.

[0084] However, from the relative positioning of the sprung pin 82 inthe channel 86 as illustrated in FIG. 6b, such relative rotation willonly continue until the pin 82 moves to the datum set positionillustrated in FIGS. 7a and 7 b of the drawings, and as described below.As the mass 22 rotates relative to the stabiliser 20, the sprung pin 82travels along the slot 88 until it encounters a ramp 92 which isprofiled to move the pin radially inwardly and then downwardly into anotch 94 defined in the lower wall of the channel 86, under theinfluence of the spring 84. In this position the pin 82 rotationallylocks the mass sleeve 22 a relative to the stabiliser sleeve 20 b.

[0085] Furthermore, the downward movement of the sleeve 62 brings thesleeve teeth 70 out of engagement with the drive ring dogs 72. Thus, onreaching the datum set position, rotation of the mandrel 14 is nottransferred to the drive ring 44, and thus is not transferred to themass 22 or stabiliser 20.

[0086] Reaching the datum set position may be identified on surface toprovide an explicit indication to the operator that the stabiliser 20 isin a known predetermined orientation relative to the mass 22. However,simply by turning of the mud pumps and then rotating the drill string inexcess of one hundred and sixty times, an operator may be assured thatthe datum set position has been achieved.

[0087] If the operator then turns the drilling fluid pumps on and thenoff again, the sleeve 62 will first be moved upwards, bringing the pin82 out of the notch 94, the pin 82 then being prevented from re-enteringthe notch 94 on the pumps being shut down once more by engagement withan edge of the ramp 92, and this relative position is illustrated inFIG. 8a of the drawings. In this configuration the sleeve 62 is oncemore engaged with the drive ring 44, and the mass sleeve 22 a is free torotate relative to the stabiliser sleeve 20 b, such that rotation of themandrel 14 will be translated to rotation of the mass sleeve 22 a andthus rotation of the mass 22. With the gear ratio as described above,forty rotations of the mandrel 14 will result in the mass 22 beingrotated through 90° relative to the stabiliser 20.

[0088] Once the mass 22 and the stabiliser 20 are at the desiredrelative orientation, the drilling fluid pumps are then turned on oncemore and the sleeve 62 returns to the configuration as illustrated inFIG. 5a, in which the stabiliser 20 and mass 22 are fixed rotationallyrelative to one another.

[0089] As noted above, the gear assembly 26 allows the operator to workfrom a datum set position to achieve a desired relative orientationbetween the mass 22 and the stabiliser 20 by rotating the mandrel 14 aknown number of times. Further, the apparatus 10 includes a sensor whichprovides an indication of the position of the stabiliser offsettingportion 28 at any point, as will now be described with reference to FIG.9 of the drawings. This figure illustrates the stabiliser offsettingportion 28, and located in an isolated chamber in the portion is aprinted circuit board (PCB) 96 including appropriate orientationsensors. The output from the sensors is passed through a high pressureelectrical connection 98 to a stationary coil 100 which is positionedaround a rotating coil 102 provided on the mandrel 14. A protected cable104 runs from the coil 102, through the mandrel bore 64, to an Mwbconnector stinger further up the drill string 12. As is well known tothose of skill in the art, an MWD tool can translate a sensor input topressure pulses in the drilling fluid, which may be detected at surface,and translated to provide an indication of stabiliser offset position.

[0090] In the event of a change in orientation of the mass 22 and offsetstabiliser 22, the drill bit 18 is urged in the opposite lateraldirection to the offset location. This may produce a ledge in thedrilled bore as the bit 18 cuts laterally to accommodate the new offset.However, any such ledge is removed by the following rotating stabiliser16; the stabiliser 16 is of similar external dimensions to the offsetstabiliser 20 (typically ⅛″ undergauge), and thus cuts a “hole” whichwill accommodate the stabiliser 20.

[0091] It will be apparent to those of skill in the art that theabove-described apparatus 10 provides directional drilling apparatuswhich permits bore projectory to be varied and controlled while stillrotating the drill string, and which allows changes in bore trajectoryto be implemented in relatively straightforward manner from surface.

[0092] Those of skill in the art will also recognise that the apparatus10 has been illustrated in somewhat simplified form, and that, in theinterest of brevity, features such as pressure compensation pistons, asknown to those of skill in the art, have not been illustrated ordescribed.

[0093] It will further be apparent to those of skill in the art that theabove-described embodiment is merely exemplary of the present invention,and that various modifications and improvements may be made thereto,without departing from the scope of the invention. For example, theabove-described embodiments feature stabilisers having helical blades:other embodiments of the invention may feature straight or axial blades.Also, the above embodiments include stabiliser bearing portions mountedvia needle roller bearings, and of course other bearing forms may beutilised, including other forms of roller bearings or plain bearings.

1. Directional drilling apparatus for use in drilling a deviated bore,the apparatus comprising: a mandrel for mounting to a drill string andhaving a main axis; a non-rotating mass rotatably mounted on the mandreland having a centre-of-gravity spaced from the mandrel axis; and anoffsetting arrangement including a non-rotating offsetting portionrotatably mounted on the mandrel, coupled to the mass, and having anouter profile defining an offset relative to the mandrel axis, and abearing portion rotatably mounted on the offsetting portion.
 2. Theapparatus of claim 1, wherein the bearing portion is mounted on theoffsetting portion via a bearing.
 3. The apparatus of claim 2, whereinthe bearing portion is mounted on the offsetting portion via a rollerbearing.
 4. The apparatus of claim 1, 2 or 3, wherein the bearingportion defines a plurality of blades.
 5. The apparatus of claim 1, 2, 3or 4, wherein the bearing portion is annular.
 6. The apparatus of any ofthe preceding claims, in combination with a drill bit.
 7. The apparatusof any of the preceding claims, wherein the non-rotating offsettingportion is coupled to the mass to permit variation of the relativeangular orientation of the mass and offsetting portion.
 8. The apparatusof claim 7, wherein the coupling between the non-rotating offsettingportion and the mass is arranged such that changes in the relativeangular orientation of the mass and offsetting portion may be effectedby rotation of one of the mass and offsetting portion relative to theother on the mandrel.
 9. The apparatus of claim 8, wherein the couplingis arranged such that said relative rotation may be achieved by rotationof the mandrel.
 10. The apparatus of claim 9, wherein a selectivelyengageable step-down gear arrangement is provided between the mandreland one of the mass and offsetting portion.
 11. The apparatus of claim10, wherein the gear arrangement comprises a harmonic drive.
 12. Theapparatus of claim 10 or 11, wherein the gear arrangement is adapted tobe selectively engaged and disengaged in response to a predeterminedcondition or signal.
 13. The apparatus of claim 12, wherein the geararrangement is responsive to fluid pressure.
 14. The apparatus of claim12, wherein the gear arrangement is responsive to relatively high fluidpressure, as experienced during the course of a drilling operation, todisengage the gear arrangement.
 15. The apparatus of any of claims 10 to14, wherein the gear arrangement is configured to be locked out whiledisengaged, to prevent inadvertent relative rotation of one of the massand offsetting portion.
 16. The apparatus of any of claims 7 to 15,wherein the coupling between the offsetting portion and the massprovides for a datum set position, in which the relative positioning ofthe offsetting portion and the mass is known.
 17. The apparatus of claim16, provided in combination with a gear arrangement between the mandreland at least one of the offsetting portion and the mass, wherein thegear arrangement is adapted to disengage on the offsetting portion andmass reaching the datum set position.
 18. The apparatus of claim 17,wherein the gear arrangement is adapted to be subsequently engageable topermit the angular orientation of the mass and offsetting portion to beset with reference to said datum set position.
 19. The apparatus of anyof claims 10 to 18, wherein the gear arrangement is adapted tosubsequently engage in response to a pressure pulse.
 20. The apparatusof any of the preceding claims, further comprising an orientation sensorfor the offsetting portion, which sensor is adapted to transmit signalsto surface indicative of offsetting portion position.
 21. The apparatusof any of the preceding claims, wherein the mandrel is hollow to permitpassage of drilling fluid therethrough.
 22. The apparatus of any of thepreceding claims, wherein the non-rotating offsetting portion comprisesa plurality of parts.
 23. The apparatus of claim 22, wherein thenon-rotating offsetting portion comprises inner and outer parts, eachpart defining an offset bore, such that by varying the relativeorientation of the offset bores it is possible vary the offset of themandrel relative to the outer profile of the offsetting portion.
 24. Theapparatus of claim 23, wherein the non-rotating offsetting portion isconfigurable such that said portion does not create an offset, allowinga bore to be drilled straight ahead.
 25. The apparatus of any of thepreceding claims, wherein a relatively flexible string portion islocated adjacent said offsetting arrangement.
 26. The apparatus of claim25, wherein said relatively flexible string portion is-a flex joint. 27.The apparatus of any of the preceding claims, wherein the mass isselected to describe a smaller diameter than the bore to be drilled. 28.The apparatus of claim 27, wherein the mass is mounted between twolarger diameter string elements.
 29. The apparatus of claim 28, whereinthe string elements are stabilisers.
 30. The apparatus of claim 28 or29, wherein one of the string elements comprises the offsettingarrangement.
 31. The apparatus of claim 28, 29 or 30, wherein both ofthe string elements are non-rotating.
 32. The apparatus of any of claims28 to 31, wherein the offsetting arrangement is adapted to be locatedbetween the mass and a drill bit.
 33. The apparatus of claim 32, whereina primary offsetting arrangement is adapted to be located between themass and the drill bit, and the other larger diameter string element atthe opposite end of the mass also provides an offset.
 34. The apparatusof claim 33, wherein the offset provided by said other larger diameterstring element is arranged to locate the mandrel and drill stringtowards the low side of the bore.
 35. The apparatus of any of thepreceding claims, wherein a rotating cutting arrangement is provided andis adapted for location adjacent a drill bit.
 36. The apparatus of claim35, wherein the rotating cutting arrangement is a rotating stabiliserprovided with cutting faces.
 35. The apparatus of claim 35 or 36,wherein the rotating cutting arrangement is adapted to be locatedbetween the first non-rotating element and the drill bit.
 36. Adirectional drilling method for drilling a deviated bore, the methodcomprising the steps: providing apparatus comprising a mandrel having amain axis, a non-rotating mass rotatably mounted on the mandrel andhaving a centre-of-gravity spaced from the mandrel axis, and anoffsetting arrangement including a non-rotating offsetting portionrotatably mounted on the mandrel, coupled to the mass, and having anouter profile defining an offset relative to the mandrel axis, and abearing portion rotatably mounted on the offsetting portion; couplingone end of the mandrel to a drill string and another end of the mandrelto a drill bit; running the apparatus into an inclined bore on thestring, the bore having a low side; orienting the offsetting portionrelative to the mass; and rotating the string, whereby the mass tendstowards an orientation with the centre-of-gravity positioned towards thelow side of the bore and thus tends to maintain the offsetting portionin a desired relative orientation in the bore, the bearing portionrotationally isolating the offsetting portion from the bore wall. 37.Directional drilling apparatus for use in drilling a deviated borehaving a low side, the apparatus comprising: a mandrel for mounting to adrill string and having a main axis; a primary offsetting arrangementincluding a non-rotating offsetting portion rotatably mounted on themandrel for inducing an offset in the mandrel axis relative to the boreaxis; and a secondary offsetting arrangement including a non-rotatingportion coupled to the primary offsetting arrangement and arranged tooffset the mandrel towards the low side of the bore.
 38. A method ofdrilling a deviating bore, the method comprising the steps: mounting adrill bit on a drill string and locating the string in a bore having alow side; rotating the drill string such that the bit rotates; providinga primary non-rotating offsetting arrangement towards the leading end ofthe string; offsetting the leading end of the string in the bore suchthat the bit tends to advance in the direction of the offset; providinga secondary non-rotating offsetting arrangement on a portion of thedrill string following said leading end and coupled to said primarynon-rotating offsetting arrangement; and offsetting said followingportion of the drill string towards the low side of the bore such thatthe mass of said portion of the string tends to maintain the offset ofsaid leading end.
 39. Directional drilling apparatus for use in drillinga deviated bore, the apparatus comprising: a mandrel for mounting to adrill string and having a main axis; a non-rotating mass rotatablymounted on the mandrel and having a centre-of-gravity spaced from themandrel axis; an offsetting arrangement including a non-rotatingoffsetting portion rotatably mounted on the mandrel and having an outerprofile defining an offset relative to the mandrel axis; means forselectively coupling the offsetting portion to the mass; and a geararrangement for selectively coupling at least one of the mass oroffsetting portion to the mandrel, whereby rotation of the mandrel istranslatable to a lesser degree of rotation of one of the mass oroffsetting portion.
 40. The apparatus of claim 39, wherein the geararrangement comprises a harmonic drive.
 41. Directional drillingapparatus for use in drilling a deviated bore, the apparatus comprising:a mandrel for mounting to a drill string and having a main axis; a drillbit coupled to a leading end of the mandrel; a non-rotating massrotatably mounted on the mandrel and having a centre-of-gravity spacedfrom the mandrel axis; an offsetting arrangement including anon-rotating offsetting portion rotatably mounted on the mandrel andhaving an outer profile defining an offset relative to the mandrel axis,the offsetting portion being coupled to the mass; and a near-gaugerotating cutter located between the drill bit and a leading non-rotatingelement of the apparatus, whereby, in use, the cutter removes ledgescreated by changes in direction of the drill bit.
 42. Directionaldrilling apparatus for use in drilling a deviated bore, the apparatuscomprising: a mandrel for mounting to a drill string and having a mainaxis; a non-rotating mass rotatably mounted on the mandrel and having acentre-of-gravity spaced from the mandrel axis; an offsettingarrangement including a non-rotating offsetting portion rotatablymounted on the mandrel and having an outer profile defining an offsetrelative to the mandrel axis, the offsetting portion being coupled tothe mass; and a offsetting portion position sensor providing an offsetposition output signal.